Borane. electrophilicity reaction with alkenes

This. section dc.scribcs the special characteristics and utility of the reaction between an alkcnc and a horane, a molecule containing B-H bonds. Boranes are electrophilic. and their reactions with alkenes are therefore very reasonable (note carefully. however, that horohydrides like BHj are different—they are anions, not electrophiles, and don t react with alkenes). 

Hydroboration is highly regioselective and stereospecific. The boron becomes bonded primarily to the less-substituted carbon atom of the alkene. A combination of steric and electronic effects works to favor this orientation. Borane is an electrophilic reagent. The reaction with substituted styrenes exhibits a weakly negative p value (-0.5).156 Compared with bromination (p+ = -4.3),157 this is a small substituent effect, but it does favor addition of the electrophilic boron at the less-substituted end of the double bond. In contrast to the case of addition of protic acids to alkenes, it is the boron, not the hydrogen, that is the more electrophilic atom. This electronic effect is reinforced by steric factors. Hydroboration is usually done under conditions in which the borane eventually reacts with three alkene molecules to give a trialkylborane. The... 

Hydroboration is highly regioselective and is stereospecilic. The boron becomes bonded primarily to the less substituted carbon atom of the alkene. A combination of steric and electronic effects work together to favor this orientation. Borane is an electrophilic reagent. The reaction with substituted styrenes exhibits a weakly negative p value... 

One of the simplest classes of nucleophiles that attacks borane is that of alkenes. The result, described as hydroboration, is an overall addition of borane across the double bond. Unlike most electrophilic additions to alkenes that occur in a stepwise manner via charged intermediates (Chapter 20), this addition is concerted so that both new bonds are formed more or less at the same time. The result is a new borane in which one of the hydrogen atoms has been replaced by an alkane. This monoalkyl borane (RBH2) is now able to undergo addition with another molecule of the alkene to produce a dialkyl borane (R2BH) which in turn undergoes further reaction to produce a trialkyl borane (R3B). All these boranes have a vacant p orbital and are flat so that repeated attack to produce the trialkyl borane is easy and normal if an excess of alkene is present. 

We now turn to the stereochemistry governed by a ring system, and we shall look at both nucleophilic and electrophilic attack, since usually they have similar stereochemical preferences rather than contrasting preferences. In addition to several reactions that are straightforwardly electrophilic attack, we shall see several which can be described as electrophilic in nature, like the reactions of alkenes with osmium tetroxide, with peracids, with some 1,3-dipoles, and with boranes, and of dienes with dienophiles in Diels-Alder reactions. Some of these reactions are pericyclic, the pericyclic nature of which we shall meet in Chapter 6. For now, it is only their diastereoselectivity that will concern us. 

The reaction of alkenes with borane, monoalkyl and dialkylboranes leads to a new organoborane (see 15-16). Treatment of organoboranes with alkaline H2O2 oxidizes trialkylboranes to esters of boric acid." This reaction does not affect double or triple bonds, aldehydes, ketones, halides, or nitriles that may be present elsewhere in the molecule. There is no rearrangement of the R group itself, and this reaction is a step in the hydroboration method of converting alkenes to alcohols (15-16). The mechanism has been formulated as involving initial formation of an ate complex when the hydroperoxide anion attacks the electrophilic boron... 

Stereochemical information is built into reagents 9 that have a nucleophilic vinyl-metal bond in the plane of the alkene. This bond already has a stereochemistry - it is cis to R1 and trans to R2 -and this information can be passed to the products providing that the reaction with electrophiles is stereospecific. In most of the methods in this chapter vinyl metals 9 M = Li, Mg, Cu, Sn, Al, Zr, or compounds closely related to them such as vinyl boranes 10 or vinyl silanes 11, react with electrophiles E+ with retention of configuration 12. 

An atom or a molecule does not have to be positively charged to be an electrophile. Borane (BH3), a neutral molecule, is an electrophile because boron has only six shared electrons in its valence shell. Boron, therefore, readily accepts a pair of electrons in order to complete its octet. Thus, alkenes undergo electrophilic addition reactions with borane serving as the electrophile. When the addition reaction is over, an aqueous solution of sodium hydroxide and hydrogen peroxide is added to the reaction mixture, and the resulting product is an alcohol. The addition of borane to an alkene, followed by reaction with hydroxide ion and hydrogen peroxide, is called hydroboration-oxidation. The overall reaction was first reported by H. C. Brown in 1959. 

Thus, the regiochemistry of hydroboration is predicted by the same general rule that applies to all electrophilic additions to alkenes The reaction of an electrophile with a carbon-carbon double bond occurs preferentially via the transition state in which a partial positive charge develops on that carbon atom better able to accommodate it. Geometric constraints inherent in the cyclic transition state 60 require that the addition of borane to the alkene proceed so that both the boron and the hydrogen add from the same face of the double bond, a process called sy -addition. 

C-Alkylations have been performed with both support-bound carbon nucleophiles and support-bound carbon electrophiles. Benzyl, allyl, and aryl halides or triflates have generally been used as the carbon electrophiles. Suitable carbon nucleophiles are boranes, organozinc and organomagnesium compounds. C-Alkylations have also been accomplished by the addition of radicals to alkenes. Polystyrene can also be alkylated under harsh conditions, e.g. by Friedel-Crafts alkylation [11-16] in the presence of strong acids. This type of reaction is incompatible with most linkers and is generally only suitable for the preparation of functionalized supports. Few examples have been reported of the preparation of alkanes by C-C bond formation on solid phase, and general methodologies for such preparations are still scarce. 

Borane, as a solution in tetrahydrofuran or generated in situ by the reaction of a metal hydride with boron trifluoride etherate, adds readily to alkenes to yield trialkylboranes. With a terminal alkene the reaction is highly (though not completely) regioselective and gives a primary trialkylborane, since the mode of addition results from the electrophilic character of the boron atom. 

No matter which of the electrophilic methods of double-bond shifting is employed, the thermodynamically most stable alkene is usually formed in the largest amount in most cases, although a few anomalies are known. However, an indirect method of double-bond isomerization us known, leading to migration in the other direction. This involves conversion of the alkene to a borane (15-16), rearrangement of the borane (18-11), oxidation and hydrolysis of the newly formed borane to the alcohol 17 (see 12-31), and dehydration of the alcohol (17-1) to the alkene. The reaction is driven by the fact that with heating the addition of borane is reversible, and the equilibrium favors formation of the less stericaUy hindered borane, which is 16 in this case. 

To understand why the hydroboration-oxidation of propene forms 1-propanol, we must look at the mechanism of the reaction. The boron atom of borane is electron deficient, so borane is the electrophile that reacts with the nucleophilic alkene. As boron accepts the rr electrons and forms a bond with one carbon, it donates a hydride ion to the other carbon. In all the addition reactions that we have seen up to this point, the electrophile adds to the alkene in the first step and the nucleophile adds to the positively charged intermediate in the second step. In contrast, the addition of the electrophilic boron and the nucleophilic hydride ion to the alkene take place in one step. Therefore, an intermediate is not formed. 

Examples of electrophilic addition of secondary phosphines to alkenes or alkynes were described. [114, 124, 125, 135]. Glueck [124-126] reported enantioselective tandem reaction of alkylated/arylation of primary phosphines catalyzed by platinum complex, proceeding with formation of chiral phosphaace-naphthenes. Palladium-catalyzed hydrophosphination of alkynes 219 tmder kinetic resolution conditions gave access to 1,1-disubstituted vinylphosphine boranes 220. However, despite screening several chiral ligands, temperatures, and solvents, the... 

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